What's a thumb button? Is that like the index finger button?
I configure my extra buttons on my device software. The Itch app is a browser, does it not heed the regular browser buttons? I do not use the App, since the search on the App does not actually work. And it is clumsy and irritating seeng a login button on the Itch page, while being logged in in the App.
So I just checked, it heeds the keystrokes for chrome it seems. But not all of them. ctrl +- does not work. But alt left/right for back and forward in history do work.
As for mass installing, not everything should be copied from Steam, but Steam does not have such a feature. If you buy two things you need to install two things. A bundle is only a clerical entity to aquire more than one thing. I think the only difference is with DLC. There are no features on Itch that support DLC directly.